Recent breeding experience improves egg ejection behaviour

Recognizing one’s own eggs is crucial for birds, especially for hosts of
brood parasites that must identify and reject different-looking parasitic
eggs. While birds seem to possess a ‘template image’ of their eggs,
whether it is innate or refined over time remains unclear. We addressed
this question by experimentally inserting either artificial mimetic eggs
(ME) or non-mimetic eggs (NME) into the nests of barn swallows (Hirundo
rustica) during the pre-laying stage. We harnessed the potential of our
individually marked population to perform a unique comparison between
naïve first-time breeders and experienced females, as well as females with
‘old’ (from a previous season) and ‘recent’ (from the previous breeding
attempt within the same season) experience, allowing us to investigate the
role of memory. We found that neither naïve nor experienced females
ejected NME more often than ME, suggesting that the template image does
not play a primary role in egg recognition. Instead, awareness of own
egg-laying might be the crucial mechanism at play, facilitating nest
sanitation behaviour. Lastly, we provide the first evidence that this
mechanism improves with recent breeding experience.